Direct fissile assay of enriched uranium using random self-interrogation and neutron coincidence response

Abstract

Apparatus and method for the direct, nondestructive evaluation of the 235U nuclide content of samples containing UF6, UF4, or UO2 utilizing the passive neutron self-interrogation of the sample resulting from the intrinsic production of neutrons therein. The ratio of the emitted neutron coincidence count rate to the total emitted neutron count rate is determined and yields a measure of the bulk fissile mass. The accuracy of the method is 6.8% (1 sigma ) for cylinders containing UF6 with enrichments ranging from 6% to 98% with measurement times varying from 3-6 min. The samples contained from below 1 kg to greater than 16 kg. Since the subject invention relies on fast neutron self-interrogation, complete sampling of the UF6 takes place, reducing difficulties arising from inhomogeneity of the sample which adversely affects other assay procedures.

What we claim is: 
     
       1. An apparatus for determining the quantity of fissile nuclides contained in a sample having substantially one type of fissile nuclide in the presence of nuclides which spontaneously generate alpha particles and nuclides which interact with alpha particles to produce neutrons, by random self-interrogation and neutron coincidence response, said apparatus comprising in combination: a. a plurality of neutron detectors disposed in a substantially cylindrical manner around the cylindrical sample under investigation so that a fraction of neutrons escaping from the cylindrical sample as a result of fission processes in the fissile nuclides, (α,n) processes in other nuclides, and from less important sources of neutrons are received and detected by said plurality of neutron detectors so as to produce signals which represent individual neutrons having been detected;   b. means for receiving said signals produced from said plurality of neutron detectors and for recording the total number of neutrons exiting the sample and reaching said plurality of neutron detectors and the total number of neutrons reaching said plurality of neutron detectors in coincidence with a chosen time interval said means being used to provide the ratio of the number of neutrons reaching said plurality of neutron detectors in coincidence within the chosen time interval to the total number of neutrons reaching said plurality of neutron detectors, wherein said ratio is related substantially to the quantity of fissile material present in the sample under investigation.   
     
     
       2. The apparatus as described in claim 1, wherein a cylindrical cadmium sheet means is inserted between the sample and said plurality of neutron detectors so that the majority of the slow neutrons which would otherwise be returned to the sample as a result of backward scattering from said plurality of neutron detectors and which would cause fissions to occur in the fissile nuclides located substantially near to the surface of the sample are absorbed, said sheet means permitting the ratio of the number of neutrons reaching said plurality of neutron detectors in coincidence within the chosen time interval to the total number of neutrons reaching said plurality of neutron detectors to be corrected for variations in density of the sample under investigation and for samples which have not been completely filled. 
     
     
       3. The apparatus as described in claim 2, wherein said plurality of neutron detectors includes several banks of staggered polyethylene-moderated  3  He counters disposed in a substantially coaxial cylindrical configuration. 
     
     
       4. The apparatus as described in claim 3, wherein said time interval is between 10 μs and 100 μs. 
     
     
       5. The apparatus as described in claim 4, wherein reflector means are placed above and below the sample in order to reflect neutrons back into the sample so as to improve the self-interrogation process. 
     
     
       6. A method for determining the quantity of fissile nuclides contained in a sample having substantially one type of fissile nuclide in the presence of nuclides which spontaneously generate alpha particles and nuclides which interact with alpha particles to produce neutrons, said method comprising the steps of: a. detecting a major portion of the neutrons emitted from the sample as a result of (α,n) reactions so as to produce a signal which is proportional to the number of neutrons detected;   b. detecting a major portion of the neutrons emitted from the sample occurring in pairs within a chosen time period so as to produce a signal which is proportional to the number of neutron pairs detected; and   c. forming the ratio of the signal which is proportional to the number of neutron pairs to the signal which is proportional to the number of neutrons detected, said ratio having substantial relationship to the quantity of fissile material contained in the sample under investigation.   
     
     
       7. The method as described in claim 6, wherein a substantial number of the neutrons are returned to the sample under investigation, wherein steps a to c of claim 6 are repeated, and wherein the resulting ratio is corrected by the signal produced in step a of claim 6 performed without said step of returning a substantial number of the neutrons to the sample under investigation, wherein the corrected ratio is more closely related to the quantity of fissile material contained in the sample under investigation than that formed according to claim 6. 
     
     
       8. The method as described in claim 7, wherein said time period is between 10 μs and 100 μs.